A Doherty amplifier includes a main radio frequency (RF) amplification circuit coupled in parallel with a peaking RF amplification circuit to provide amplification to an RF signal. At lower power levels, the main RF amplification circuit in the Doherty amplifier is turned on and biased for linear operation, while the peaking RF amplification circuit is turned off. However, the peaking RF amplification circuit turns on once the RF signal reaches a particular signal level, which is generally at or near a compression point of the main RF amplification circuit. The main RF amplification circuit is held at or near the compression point while the peaking RF amplification circuit provides additional amplification. Unfortunately, typical Doherty amplifiers rely entirely on the peaking amplifier bias level to turn on the peaking RF amplification circuit. While the main RF amplification circuit of the typical Doherty amplifier is generally provided to have a linear Class AB amplifier topology, the peaking RF amplification circuit of typical Doherty amplifiers relies on a non-linear Class C amplifier topology to remain in cutoff at signal levels below the compression point of the main RF amplification circuit. Although the input power threshold at which the peaking amplifier begins to operate can set by the Class C bias level, the rate at which the non-linear peaking amplifier turns on above the threshold level is uncontrolled. This results in non-linear behavior and a dip in the power efficiency of the typical Doherty amplifier.
Accordingly, RF circuit designs that improve the performance of the Doherty amplifier are needed.